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Missing links Fossil evidence of the earliest monkey and Old World ape has been found in the East African Rift Valley.

The discovery of the 25.2 million-year-old fossilised partial jaw and tooth suggest tectonic movement could have played a role in human evolution, say researchers.

The study, by an international team of scientists led by Dr Nancy Stevens from Ohio University, is published today in the journal Nature.

DNA evidence suggests the primate group that includes apes and humans (hominoids) diverged from Old World monkeys, such as baboons and macaques, between 25 and 30 million years ago.

Until now, however, fossil evidence from the East African Rift, considered the cradle of humanity, has been only around 20 million years old.

But the latest finds, discovered in precisely dated 25.2 million-year-old rocks in the Rukwa Rift in Tanzania, correspond to the dates indicated by DNA evidence, says research team member Dr Eric Roberts of James Cook University in Townsville.

"The fossils provide a link between the earlier primates and the later group that gave rise to apes and humans," says Roberts. "That's what makes this paper fairly significant."

Landscape shaping human evolution

The new skull fragments belong to a previously unknown monkey named Nsungwepithecus gunnelli, and an ape dubbed Rukwapithecus fleaglei.

From the ape, the researchers unearthed a lower jawbone with several teeth, while for the monkey the record is more sparse with a much smaller piece of jawbone holding just one tooth.

Roberts, who was the geologist on the team, says the findings suggest the movement of tectonic plates helped shape human evolution.

The East African Rift runs from the Red Sea in Arabia, through Ethiopia, down through East Africa and is responsible for creating the great rift lakes of Africa.

"It's one of the greatest geologic landscape features on the planet," says Roberts. "It results from the continent pulling apart."

To date most important fossils have been found in the eastern branch of the rift, but none in rocks older than 20 million years old.

But last year Roberts and colleagues found that the Tanzanian section of the western branch of the rift was as old as 26 or 30 million years old -- much older than previously accepted.

This told the researchers that the rift began at least 25 million years ago which would have created a lot of environmental change and possibly climate change, says Roberts.

"When you change the landscape you are creating different environments and creating a situation that favours evolution and diversification," he says.

All this was occurring around the time of the evolution of the newly discovered ape and Old World monkey.

"The landscape change may have been one of the drivers of this evolution," concludes Roberts.